(1) Field of the Invention
The present invention relates to a non-magnetic mono-component developing device which supplies, in the correct manner, a non-magnetic mono-component toner to the photoreceptor surface, by forming a thin layer of the toner on the developing roller surface. More detailedly, the present invention relates to a non-magnetic mono-component developing device which, upon development, creates a thin layer of the toner by regulating the amount of toner adhering to the peripheral surface of the developing roller, with a doctor blade which is arranged with its free end directed toward the upstream side with respect to the rotational direction of the developing roller.
(2) Description of the Prior Art
Generally, an electrophotographic image forming apparatus develops an electrostatic latent image formed on the photoreceptor surface by making use of the photoconductive effect, into a visual toner image, which in turn is transferred onto a recording medium, thus forming an image. Therefore, the image forming apparatus includes a developing device for supplying the toner (developer) to the static latent image formed on the photoreceptor surface.
The developing device is comprised of a developing roller and a toner storage hopper. The developing roller is configured so as to rotate and arranged so that its peripheral surface opposes the photoreceptor surface. The developing roller bears the toner in the storage hopper on its peripheral surface and supplies it to the photoreceptor surface. In particular, in a developing device for supplying a non-magnetic mono-component toner to the photoreceptor surface, a doctor blade is arranged on the peripheral surface of the developing roller. This doctor blade is put in sliding or pressing contact with the peripheral surface of the developing roller with the toner held therebetween, so as to regulate the thickness of the toner layer over the peripheral surface.
In the non-magnetic mono-component developing device, the condition of toner supply to the photoreceptor surface changes depending on the thickness of the toner layer attached on the peripheral surface of the developing roller. The thickness of the toner layer is determined by the state of abutment (including the state of proximity) between the peripheral surface of the developing roller and the doctor blade. This means that image forming is dominated by the state of abutment of the doctor blade against the peripheral surface of the developing roller.
In a developing device disclosed in Japanese Patent Application Laid-open Hei 5 No.323778, the distal part of the doctor blade is bent at an angle ranging from 0xc2x0 C. to 90xc2x0 in a direction away from the developing roller surface. This doctor blade is extended approximately along the tangent of the roller surface so that the angled distal part is located on the downstream side with respect to the developing roller""s direction of rotation. In this arrangement, the doctor blade abuts the peripheral surface of the developing roller so as to adjust the toner layer to the correct thickness. The disclosure of this conventional doctor blade, however, has no reference to the influence from the R-value(radius of curvature) at the bent portion.
Japanese Patent Application Laid-open No.2001-92248 discloses a doctor blade configuration in which the distal part of the doctor blade is bent at an angle equal to or greater than 90xc2x0 so that the bent portion regulates the toner. In this disclosure, the suitable range of the R-value of the bent portion is specified to be from 0.25 to 0.45 mm.
In the developing device using a non-magnetic mono-component toner, the behavior of the toner as a granular material exerts a large influence on formation of the toner layer. In a configuration in which the powdery toner layer is regulated by the bent portion, not only the R-value of the bent portion but also the surface properties (burrs and the like formed during bending) produce an effect on formation of the toner layer or formed images. In order to solve this problem, post-processes such as polishing the bent portion may and should be implemented, but this means addition of extra steps, leading to increase in cost.
In order to avoid the influence from the bent portion, it is possible to arrange the blade in such a manner that the blade comes into surface contact with the developing roller with its distal part extended to the upstream side with respect to the developing roller""s direction of rotation. However, an excessive extension of the blade causes an excessive amount of toner to enter the space between the extension and the developing roller and push up the blade, resulting in degradation of the regulating force of the blade, which may cause difficulties in the uniform electrification of the toner and formation of the thin layer.
Further, though the optimal extension of the blade will also vary depending on the developing roller""s diameter and the toner particle size, there have been no conventional developing devices which refer to this point. Hence, a solution to the problem of a uniformly charged thin layer of non-magnetic mono-component toner being unable to be maintained in a continuous manner has been awaited.
The present invention has been devised in order to solve the above problems, it is therefore an object of the present invention to provide a non-magnetic mono-component developing device which can create a toner layer having a uniform thickness with uniform static charge characteristics across the full range of the image forming area on the peripheral surface of the developing roller so as to form images of good quality.
The inventor hereof closely investigated and examined the relationship between the bent position of the doctor blade, the developing roller""s diameter and the mean particle size of the toner, and found an appropriate relationship between these factors which enable the thickness of the toner layer on the peripheral surface of the developing roller and the charge-to-mass ratio of the non-magnetic mono-component toner to be set at correct values and which enable a correct amount of toner having proper static charge characteristics to be supplied to the photoreceptor surface, and finally has completed the present invention.
Specifically, the non-magnetic mono-component developing device according to the present invention is characterized by the following configurations or the following means and gives a solution to the above problems.
(1) A non-magnetic mono-component developing device, comprising:
a developing roller for supplying non-magnetic mono-component toner layered on the peripheral surface thereof to the photoreceptor surface as it being rotated; and
a doctor blade which is cantilevered at its proximal end, has a distal end as a free end, extended to the upstream side with respect to the direction of rotation of the developing roller and can regulate the thickness of the toner layer by abutting part of the side flat surface of the distal end portion against the peripheral surface of the developing roller, characterized in that the doctor blade has, at its distal end, an extension projected in the tangent direction from the abutment S and a bent portion (bent angle xcex8) angled so as to be positioned away from the peripheral surface, and Z is specified so as to satisfy the following relation:
0 less than Zxe2x89xa6[(Rr+4xc3x97xcfx86t)2xe2x88x92Rr2]1/2,
where Z represents the length of the extension in millimeters from the abutment point S to the bent portion, Rr the radius of the developing roller in millimeters, and xcfx86t the mean particle size of the toner in millimeters.
Here, xe2x80x98abutmentxe2x80x99 should not be limited to the case where the blade comes into pressing contact with the roller with the toner held therebetween, but may include the case where the blade is arranged along and in indirect contact with the roller via the toner. That is, as long as the doctor blade can regulate the toner on the peripheral surface of the developing roller, the situation is considered as xe2x80x98abutmentxe2x80x99. In the present invention, the state of abutment in which the blade side is able to come into pressing contact with the roller surface with the toner held therebetween is preferred as described below.
(2) The non-magnetic mono-component developing device defined in (1), wherein the radius of curvature (R-value) at the bent portion is set within the range of 0.1 to 0.3 mm, and the length L from the bent portion to the distal end is equal to 0.5 mm or greater.
(3) The non-magnetic mono-component developing device defined in (1), wherein the relation holds:
xcex8xe2x89xa7xe2x88x922.5xc3x972Rr+113,
where xcex8 represents the bent angle of the bent portion in degrees and 2Rr the diameter of the developing roller in millimeters.